Use single shared memory segment for all shared variables in Pgpool parent process

Use single shared memory segment for all shared variables in Pgpool parent process

Shared memory segments is a limited resource and opening too many segments
should be avoided.
Also the default limit for on some operating systems like
OS/X (kern.sysv.shmmni: 32) is very small that you have to increase
shmmni just to run Pgpool-II. 
So to avoid creating too many shared memory segments, the commit creates a
single shared memory segment big enough to hold all required shared memory
and put all shared memory variables within that segment one after the other.